Abstract
Alkaline chlorination, an efficient but high chemical cost process, is commonly employed for cyanide (CNâ) removal from CN-rich wastewater streams. CNâ removal and recovery through the precipitation of Prussian Blue (Fe4III[FeII(CN)6]3, PB) or Turnbullâs Blue (Fe3II[FeIII(CN)6]2, TB) were realized using iron salts, leading to a cost-effective and sustainable process producing a valuable recovery product. However, the precipitation of PB and TB is highly affected by pH and dissolved oxygen (DO). CNâ removal and recovery from CN-containing water by crystallization of PB and/or TB were investigated using dissolved iron that was electrochemically generated from a sacrificial iron anode under various pH values, initial CNâ levels (10 to100 mg/L) and DO levels (aeration, mechanical mixing, and N2 purging). It was shown that the complexation of CNâ with Fe ions prevented the vaporization of HCN under acidic pH. At pH of 7 and initial CNâ concentration of 10 mg/L, CNâ removal efficiency increases linearly with increasing Fe:CNâ molar ratios, reaching 80% at the Fe:CNâ molar ratio of 5. A clear blue precipitate was observed between the pH range of 5â7. CNâ removal increases with increasing initial CNâ concentration, resulting in residual CNâ concentrations of 8, 7.5 and 12 mg/L in the effluent with the Fe:CNâ molar ratio of 0.8 for initial concentrations of 10, 50 and 100 mg CNâ/L, respectively. A polishing treatment with H2O2 oxidation was employed to lower the residual CNâ concentration to meet the discharge limit of <1 mg CNâ/L.
Highlights
CNâ to form FeII (CN)â is a common contaminant encountered in the wastewater streams from mining and electroplating processes
Chemical equilibrium modeling using a commercial chemical equilibrium software, MINEQL+, was first performed with the condition set at the temperature of 25 ⊠C and the initial cyanide concentration of 10 mg/L to reveal the effects of pH and Fe:CNâ molar ratio on the formation of the primary FeII (CN)6 4â and FeIII (CN)6 3â complexes involved in the precipitation of PB and TB, respectively
Blue using iron electrode was studied on synthetic CN-rich water
Summary
CNâ is a common contaminant encountered in the wastewater streams from mining and electroplating processes. Alkaline chlorination is an efficient process for the removal of cyanide to reach the discharge limit of 1 mg CNâ /L in Taiwan, the process is highly pH-dependent and associated with high chemical costs [3,4]. Blue (Fe4 III [FeII (CN)6 ]3 , PB) and/or Turnullâs Blue (Fe3 II [FeIII (CN)6 ]2 , TB) [5,6,7]. As indicated in Equation (1), ferrous ions first complex with CNâ to form FeII (CN)6 4â. Once extra ferrous is added, the precipitation of FeII 2 FeII (CN) immobilizes and removes CNâ from the solution with a Fe(II):CNâ molar ratio of 0.5, as illustrated in Equation (2). With dissolved oxygen (DO), FeII 2 FeII (CN) is oxidized to form PB as indicated in Equation (3), releasing ferrous
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